lib.utexas.edu/.../YT-Intro_30_Reaction-Stoichiometry-g-g-Method-A.mp4

Hello and welcome to
the next video

in our series on
reaction stoichiometry.

This one we will convert from grams
of one substance into

grams of another substance and
time allowing,

we will go through multiple methods
of doing so.

Alright, so the key to this is
having the correct

balanced chemical reaction and
then also being

able to calculate molecular mass
so if you do not know

how to calculate molecular weight,
then please consult

one of our other videos that
goes over that.

Alright, so how many grams of
oxygen are required

to burn 92.6 grams of propane?

In another video, we figrued out
how to break down

these words and know what
the balanced chemical reaction is

so please refer to
one of those videos.

We know that it's C3H8 plus O2
gives CO2 and H2O.

Propane is C3H8.

Alright, so I'm going to balance
the carbons,

balance the hydrogens,
and then balance the oxygens last.

Alright, now we'll look at,
before I go into

one of these methods, I want to
slow down just a minute

and write down what I'm given.

I'm given 92.6 grams of propane
and I want to find

how many grams of O2.

Alright, so let's look at the
first method here.

I'm just going to call it
method A.

That's just a reference name
to say it's different that method B

or method C.

Alright, so method A is sometimes
known as the fence method

or the train track method,
I've heard all different

kinds of names for it.

We have how many grams
of O2 for 92.6 grams of C3H8?

And I've got to squish these all in
so it may get a little messy here.

First I've got to get rid of
my grams of C3H8.

Alright, and I need to get it
into mols of C3H8.

Now what do I know that has
mols per gram

or grams per mol?

Oh yeah, molecular weight.

So I calculate that for C3H8
and I get 44.11 grams per 1 mol.

Now, when I divide out the grams,
I end up with mols at this point.

So I've gone from grams of C3H8
into mols C3H8.

Great, that's where I am so far.

If I've stopped there,
I would have mols of C3H8.

I don't want mols,
I want grams of O2

so I need to use the
mol to mol ratio,

and yes, that's the only way
to get there

to change the molecule from
C3H8 to O2 is the mol to mol ratio.

Alright, so then I'm going to use
the numbers from the

balanced chemical equation
I see in front of the O2 is a 5.

So I'm going to put that 5
there and in front

of the C3H8 is an invisible,
implied 1.

Alright, now if I were to stop there,
I would have the mols of O2.

That's still not what I need.
I need grams.

So I'm going to keep going.

I'm going to put mols of O2
on the bottom and

grams of O2 on the top and
think to myself,

is there something I know that's
grams per mol?

Yes, indeed,
molecular weight,

and it is O2, not just O
so it's 32.00 (grams) per 1 mol.

I usually go 2 places past
the decimal,

it does me well in most cases.

Alright, scrolling up just
a little bit.

My grams of O2 ends up
336 grams of O2 are needed.

And that's method A.

Let's look at another method.

Oh my goodness,
that's so much fun.

Oh, what did we do?

We went from grams of C3H8
to mols of C3H8

to mols of O2 to grams of O2.
Cool.

Alright, method B.

We're going to need to put in
our next video.

I just can't talk fast enough.

Alright, so let's spend just
a few seconds

to look at this.

We changed our grams
into mols as our first step

using molecular weights so
the first step

is to get to mols.
Alright,

then once you get to mols,
you get the mol to mol ratio

and then get to whatever
you're looking for.

In this case it's grams and
to go between mols and grams,

we always use molecular weight.

Thank you and hope you watch
the next video as well

for methods B and C.